An automatic navigation system, which is incorporated in an automobile, controls such operations as braking actuation when an obstacle is detected on the roadway ahead of the vehicle and steering actuation to follow a white guiding line which is provided along the roadway or to follow another vehicle cruising ahead. For these control operations, the detection of respective objects such as an obstacle is carried out by monitoring the forward view of the vehicle with an image sensor and by processing the data of images which are obtained in the monitoring. As such an image sensor, most popularly used is a video camera of the NTSC color system, i.e., an ordinary color video camera, whose aspect ratio is 3:4.
If the curves of the roadway are taken into consideration, a horizontal angle of view of 20 to 100 degrees is required for the monitoring of the forward view. If this horizontal angle of view is maintained by an NTSC camera, then the images obtained must include areas in the upper and lower portions thereof, e.g., skies, which are not necessary as information for the operation of the automatic navigation system. These upper and lower portions of the images constitute an unwanted load for the image processing operation of the system.
Hi-Vision camera, another type of video camera, has an aspect ratio of 9:16. If a Hi-Vision camera is applied alternatively as image sensor of the automatic navigation system, then the unwanted upper and lower portions of the images are reduced accordingly. However, for the automatic navigation system to operate safely and securely without a fail, when the vehicle approaches a curve along the roadway, the horizontal angle of view must be wide enough to take in a large section of the curvature of the roadway in the images still. In such cases, the images must include yet substantially unwanted upper and lower portions. Hi-Vision cameras generally have image sensors (or photosensitive portions) whose resolutions are higher than those of NTSC cameras, so the automatic navigation system with a Hi-Vision camera can obtain more data from the detailed images than with an NTSC camera. However, the incorporation of a Hi-Vision camera makes the system comparatively expensive.
Japanese Laid-Open Patent Publication No. H5(1993)-219444 and Japanese Laid-Open Patent Publication No. H6(1994)-205298 disclose an imaging device or an image sensor which can change the aspect ratio between 3:4 and 9:16 as the need arises. Because the change of the aspect ratio is limited to these two ratios, it is impossible for the automatic navigation system to obtain the images in an aspect ratio which is horizontally wider than 9:16. In this respect, it is possible to construct a system or mechanism which can shut out the upper and lower portions of the imaging device by applying similar technology disclosed in the above mentioned patent publications so that the image sensor can generate the images in a wider aspect ratio. However, imaging devices or image sensors constructed in such a way are substantially large in size and expensive to produce.
Furthermore, the above mentioned automatic navigation system requires a camera which is capable of setting the aspect ratio variably and appropriately for monitoring in performing various types of automatic navigation. For example, it is necessary for the camera to have a yet greater horizontal angle of view when the system expects a relatively sharp curve on the road. Thus, for the purpose of performing automatic navigation securely and safely, there has been a need for a method for setting the aspect ratio in correspondence with the condition of the roadway.